qeth: no write permission for readonly sysattr

User is not allowed to write into bridge_state sysfs file.
Fixed attribute not mislead the user

Signed-off-by: Lakhvich Dmitriy <ldmitriy@ru.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Reported-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Reviewed-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Reviewed-by: Thomas Richter <tmricht@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>